AMOC slowdown amplifies North Atlantic salinity variability to unprecedented levels

The Atlantic Meridional Overturning Circulation (AMOC), a key component of the Earth system, is projected to weaken under anthropogenic warming, but how this change affects oceanic variability remains unclear. Here we show, using a sustained global warming scenario, that AMOC weakening triggers extreme upper-ocean salinity variability in the North Atlantic, with amplitudes far exceeding historical levels. The salinity variability amplifies as it travels across the North Atlantic Ocean and is linked to a slowdown of the Gulf Stream and strengthened spatial salinity contrasts. An idealized model demonstrates that these changes in the mean ocean state promote salinity extremes, with their amplification critically dependent on the salinity–temperature coupled feedback. Once the AMOC slowdown is underway, enhanced variability can arise even under climate mitigation because ocean circulation recovers slowly. Emergent compound extremes of salinity and sea level increase the risks to coastal ecosystems and societies in Europe. The weakening of ocean circulation under global warming could drive the emergence of unprecedented North Atlantic salinity variability, with impacts persisting long after warming slows and escalating risks for European coastal ecosystems and societies.